Steel breastplate, or Stalnoi Nagrudnik (Russian: Стальной нагрудник) is a type of body armor similar to a cuirass developed by the Red Army in World War II. The native Cyrillic abbreviation for the vest was "СН", the Cyrillic letters Es and En. It consisted of two pressed steel plates that protected the front torso and groin. The plates were 2 mm (.08") and weighed 3.5 kg (7.7 lbs). This armor was supplied to SHISBr (assault engineers).

The Steel breastplates along with the conventional steel SSh-40 helmets equipped the assault engineers and demining brigades of the Supreme Command Reserve STAVKA, for which they are sometimes called "tubular infantry." Breastplate SN-42 was designed to protect against bayonet attacks, small fragments of shrapnel, and 9mm pistol bullets with lead cores, providing protection against fire from a MP-38/40 submachine gun from distances of 100–150m, and a single shot from a 7.92×57mm Mauser rifle (like the Gewehr 41), but on the condition that the bullet went on a tangent. Following the adoption of the Wehrmacht on the supply of 9mm cartridges, the cartridge code R.08 mE (German: mit Eisen Kern), with a bullet with mild steel (iron) core, required the thickness to be increased to 2.6 mm for the chest plate (2.5 - 2.7 mm). This redesign received the name SN-46.

By modern standards, they are roughly equivalent to a Class II vest.

USSR - steel breastplates SN-42 began to arrive in the army in 1942 and were later used during World War II.

Poland - Soviet steel cuirass entered service of the 1st Polish Army (as of October 31, 1944 there were 1000 pieces).

The Third Reich - by some accounts, captured Soviet steel breastplates came to supply the German army, also in Germany in limited quantities (only for parts of the SS, Mostly assault squads) produced similar breastplates.

Estimates of the plates' performance from front-line soldiers were mixed, receiving both positive and negative feedback. The vest worked well in street fighting and other type of close quarter combat. However, in the field where assault teams often had to crawl the breastplates were just an unnecessary burden.